The present invention relates to a circuit arrangement for a telephone set, comprising a voltage gain and control circuit element inserted between a first and a second line end portion, which portions are connected to a diagonal of a rectifier bridge having a telephone line connected to the other diagonal of this bridge. The gain and control circuit element has at least one modulation terminal coupled to the first line end portion, and a reference terminal coupled to the second line end portion via a line current measuring resistor element to provide a reference voltage, said telephone set further including a line current control circuit which includes a first field effect transistor and/or high-gain bipolar transistor whose main current path is located in the branch between the first and second line end portions which includes the voltage gain and control circuit element This main current path may be used to provide a voltage drop in this branch.
Such a circuit arrangement for a telephone set is known from European Patent Application No. EP-A 0 146 183, to which U.S. Pat. No. 4,626,626 corresponds, which particularly discusses the use of an integrated circuit which ensures the main functions as regards transmission, reception and actual supply voltage control, for example, an integrated circuit commercially available under the name of TEA 1060 (or its derivatives) marketed by PHILIPS.
Since the standards to be satisfied by the telephone sets are changed relatively frequently and, in addition, these standards considerably vary from one country to another, it is not possible to provide entirely integrated circuits which ensure all the basic functions of the telephone set without adding external components with which it is possible to adjust the operation so that particular regulations which are applied in the territory where the circuits are used are taken into consideration.
It is a known fact that as a result of the integration of ever more complex functions in circuits manufactured on a large scale, a considerable price reduction is achieved for the arrangements both as regards the number of components and as regards the labour cost necessary for the assembly. Since external components have to be added to an integrated circuit, as is the case in a telephone set for the reason which has just been given, there is a problem of making as sensible a selection as possible while enabling a manufacturer to minimize the quantity and price of the external components. Nevertheless, it should still be possible by means of these components to precisely determine the operating conditions desired under any particular circumstances.
Due to the fact that the length of the connecting line of a telephone set may vary considerably, the line current which appears the moment the receiver is lifted off the hook may also vary considerably and, therefore, the standards frequently require that this current be limited to a maximum value when the line is very short, so that unless and, furthermore, annoying energy loss is avoided when specific switches are in operation. The circuit that provides such a line current limitation or a current control necessitates the presence of a certain number of external components because of relatively high voltages these components may be subjected to(and, more specifically, said first transistor which produces a line voltage drop which may exceed the permissible voltage in an integrated circuit of current technology) as well as other associated components such as resistors whose value may be chosen at random to fix the current limit necessary for satisfying the standard.
The circuit arrangement described in U.S. Pat. No. 4,626,626 makes a relatively large number of external components necessary and thus leads to a structure of the telephone set whose operation is certainly technically satisfactory but for which a lower cost price would be desirable.